This invention relates to a luster detector for determining whether or not a surface of an object under test is glossy.
A photo-electric switch is an example of a detector which receives light reflected from an object under test to perform a predetermined detecting operation. FIG. 8 shows a luster detector which utilizes such a photo-electric switch to detect the luster of a surface of an object under test. The luster detector includes a light emitting section 40, a regular reflection light detecting section 42, and an irregular reflection light detecting section 44. The light emitting section 40 and the regular reflection light detecting section 42 are so positioned that the angles R1 and R2 formed by light paths L1, L2 and the normal 49 of a surface M1 are equal to each other. The irregular reflection light detecting section 44 is positioned outside the light path L2 of a ray of light reflected regularly from the surface. That is, the ray of light reflected regularly from the surface under test is applied to the regular reflection light detecting section 42, and some of the rays of light diffuse-reflected therefrom are applied to the irregular reflection light detecting section 44.
The quantities of light received by the light detecting sections 42 and 44 are applied to respective circuits to provide light detection signals. The light detection signals are applied, for instance, to a substraction circuit, so that the difference between the regularly reflected light and the diffuse-reflected light is obtained, thereby to determine the glossiness of the surface M1.
When the surface M1 is high in glossiness, the quantity of light reflected regularly therefrom and applied to the regular reflection light detecting section 42 is considerably large, and therefore the quantity of light received by the regular reflection light detecting section 42 differs greatly from the quantity of light received by the irregular reflection light detecting section 44. In the case where the surface M1 is not glossy, the quantity of light diffuse-reflected therefrom is larger, and therefore the difference between the quantities of light received by the regular reflection light detecting section 42 and the irregular reflection light detecting section 44 is small. Hence, when the difference between the quantity of light reflected regularly therefrom and the quantity of light reflected irregularly therefrom is detected using the light detection signals, it can be determined whether or not the surface M1 is glossy.
However, the above-described conventional luster detector is disadvantageous for the following reason: when a number of objects are tested one after another, sometimes their surfaces M1 are inclined or shifted. If the surface is inclined as indicated at M2 in FIG. 8, then the light path of the ray of light reflected regularly therefrom is changed to a light path L7, and if the detecting surface is shifted as indicated at M3, then the light path is changed to a light path L8. In both cases, the ray of light reflected regularly from the surface is not applied to the regular reflection light detecting section 42, and accordingly it is impossible to correctly detect both the quantity of regularly reflected light and the quantity of diffuse-reflected light; that is, it is impossible to perform the luster detection operation with high accuracy. In addition, the conventional luster detector cannot handle objects of different color because the quantity of light reflected from an object's surface under test depends on the color of the latter.